Every time (tried twice, then ran out of mana) I am defeating him in tactical (using Storm spell, not sure if the method matters), he is defeated in combat, I get XP for the kill, but then he remains in the world map with 8 hp.
Looking at his abilities, I see 'Regeneration 3' which restores 8 health / round. It looks like the 'tick' of the regen ability happens after combat ends, but before the unit is destroyed, thus preventing unit death.
